FTDI FT232R - impossible to open port

Hello,
I am using FTDI FT232R to comunicate via RS485 with Dynamixel Robotis servo (in particular EX106).
My program is written in C++ using visual C++ 2010 and I am using the last driver from FTDI (version 2.8.24.0).
I am using Windows7 (64 bit) and I am testing my program on many machine. On some machine it works and on others it doesn't. If I launch my program using "run as administrator" under Windows 7, the program doesn't work on any PCs.
In fact the error which FTDI library returns is

FTDI_INVALID_HANDLE

It seems that using "run as administrator" prevents the communication with FTDI chip
Is anyone could kindly help me?
Thanks